Pressure transducers are used in a wide range of applications. In many cases, it is desirable to measure the pressure of fluid media which may be harmful or corrosive to the transducer material, such as water, fuel, oil, acids, bases, solvents, other chemicals, and corrosive gases. There are numerous high-volume applications where a media compatible pressure transducer is highly desired but not available in any currently available technology with satisfactory durability, performance, or price characteristics. There is a need for media compatible pressure sensor packages which have substantial performance and cost advantages over existing technologies and provide new capabilities not previously realized.
Pressure is one of the most commonly measured physical variables. While pressure measuring instruments have been available for many decades, the proliferation of inexpensive solid-state silicon pressure transducers has resulted in tremendous growth in the number and different types of applications of pressure transducers. The most common pressure transducers are solid-state silicon pressure transducers employing a thin silicon diaphragm which is stressed in response to an applied pressure. The stress is measured by piezoresistive elements formed in the diaphragm. Pressure transducers are also formed similarly using metal foil diaphragms and thin film stress sensing elements. In some cases, one or two pressure sensing diaphragms are part of a parallel plate capacitor, in which the applied pressure is detected by the change in capacitance associated with the deflection of the loaded plate or plates. Other pressure measurement techniques include spring-loaded members which move in response to an applied pressure. For vacuum pressures there are a wide variety of other pressure measurement techniques.
Pressure transducers are used to measure pressures in a wide variety of fluid media, including but not limited to: air, nitrogen, industrial process gases, water, automotive fluids, pneumatic fluids, coolants, and industrial chemicals. In many important applications, the media which the pressure transducer must measure is corrosive or damaging to the transducer itself. In these cases, the pressure transducer must either be constructed in such a way that it is resistant to the media of interest, or the transducer must somehow measure the pressure while being physically isolated from the media of interest. To date, pressure sensors are either inadequately protected for media compatibility or are prohibitively expensive for many applications.
Many different types of pressure sensors have been devised. The overwhelming majority of pressure transducers for media compatibility are protected by a stainless steel housing, with a single stainless steel diaphragm providing a barrier between the pressure sensing element and the media. The empty volume between the steel diaphragm and the pressure sensing element is filled with a fluid, such as silicone oil. When the steel diaphragm deflects due to an externally applied pressure, the essentially incompressible fluid transmits that pressure to the internal pressure sensing element, which produces a voltage or current signal proportional to the pressure. While these stainless steel packaged pressure transducers are widely used, they have several shortcomings, including relative complexity and high cost. While in some industrial applications the rugged steel housing may be preferred regardless of price, there are numerous high-volume applications for media compatible pressure sensors in which the cost of the steel packages are prohibitively expensive. Also, the steel diaphragms, while thin, are inherently stiff due to the high modulus of steel. This results in a loss of sensitivity to applied pressure which is undesirable for transducer performance, especially at lower applied pressures. These types of sensors are also inherently sensitive to temperature. A temperature rise causes the internal fluid to expand. Constrained by the steel diaphragm, the pressure of the fluid rises, producing a false pressure reading. This temperature sensitivity is typically corrected with external passive or active electronic components which add to the cost of the transducer. Fourth, the stainless steel material is not satisfactory for many media applications. Stainless steel will eventually corrode in certain environments with harsh acids and bases present. In some applications, such as in the semiconductor industry and biomedical applications, even if the steel is resistant to the chemical substance in question, minute trace amounts of steel or corrosion products released into the media cannot be tolerated. Also, steel housings add substantially to the weight and size of the transducers.
Solid-state silicon pressure sensors which are not specially packaged for media compatibility are only used with air or other inert gases. Because of the shortcomings of the steel packaged sensors and the conventional silicon sensors, other kinds of packages have been devised. One approach has been to limit media exposure to the more rugged portions of the silicon sensor, allowing the media to contact the silicon diaphragm while isolating the corrosion-sensitive metal portions of the sensor. This has been most readily accomplished by allowing media to contact the backside of the silicon diaphragm only. Because differential pressure is often needed, many of these methods involve arranging two pressure sensors together so that the backsides of both are used to measure a differential pressure. U.S. Patents relating to this approach include U.S. Pat. Nos. 4,695,817; 4,763,098; 4,773,269; 4,222,277; 4,287,501; 4,023,562; and 4,790,192. These approaches provide some media compatibility improvements, but are of limited usefulness since silicon corrodes in some acid or base environments. These approaches may add substantially to the sensor cost (especially if two sensors are used for one measurement application), or may be impractical to manufacture and assemble due to the unusual component orientation, assembly, bonding, sealing, and electrical interconnection requirements. The complex assembly of some of these devices is apparent from even a casual examination of the patent drawings. Another approach to exposing the silicon diaphragm only while protecting the metal regions is described in U.S. Pat. Nos. 4,656,454 and 5,184,107. These devices employ an elastomeric seal which contacts the diaphragm and separates the diaphragm and metal interconnect regions. Again, this device provides some improvement over conventional silicon pressure sensors but the elastomeric material also has significant limitations in the chemical environments it can withstand.
Silicon pressure sensors have also been coated with a protective material, such as silicone gel, to protect the device. This approach is very limited in the types of media in which it is effective, and the coating can also affect the sensor performance. A rubber membrane diaphragm has been used instead of steel for media isolation with a fill fluid. The media compatibility of a rubber device is an improvement over bare silicon but is still limited. Molded diaphragms are disadvantageous from a manufacturing standpoint for the reason that it is difficult to obtain uniform thickness in mass production.